Thermoplastic material

ABSTRACT

A circuit package for housing semiconductor or other integrated circuit devices (“die”) includes a high-copper flange, one or more high-copper leads and a liquid crystal polymer frame molded to the flange and the leads. The flange includes a dovetail-shaped groove or other frame retention feature that mechanically interlocks with the molded frame. During molding, a portion of the frame forms a key that freezes in or around the frame retention feature. The leads include one or more lead retention features to mechanically interlock with the frame. During molding, a portion of the frame freezes in or adjacent these lead retention features. The frame includes compounds to prevent moisture infiltration and match its coefficient of thermal expansion (CTE) to the CTE of the leads and flange. The is frame is formulated to withstand die-attach temperatures. A lid is ultrasonically welded to the frame after a die is attached to the flange.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

This application relates to circuit packages for integrated circuitsand, more particularly, to circuit packages that include metal flangesand high-temperature thermoplastic frames.

Semiconductor and other integrated circuit devices (sometimes referredto as “chips” or “die”) are typically mounted inside circuit packages toprotect the die and to facilitate electrically, mechanically andthermally connecting the die to printed circuit boards, heat sinks andthe like. A typical circuit package includes a base (commonly referredto as a “slug” or “flange”), a protective insulating housing and leadsextending through the housing. Inside the housing, the leads areelectrically bonded directly, or more commonly by wires, to contacts onthe die.

The protective housing is made of a dielectric material, such as plasticor ceramic, and is attached to the flange to encapsulate the die andbonding wires and to protect them against intrusion of water vapor andother atmospheric gases. Most protective housings comprise two pieces,i.e. a set of sidewalls (a “frame”) and a lid, although some housingsare molded as one-piece assemblies. The order in which the frame and thedie are attached to the flange varies, depending on the material of theframe and, more particularly, the maximum temperature the material cantolerate without deforming or being otherwise damaged.

A circuit package for a high-power die typically includes a metalflange, to which the die is attached, often by eutectic soldering. Theflange typically provides mounting features, such as screw holes orslots, by which the circuit package can be mounted, such as to a heatsink. In use, the flange conducts heat from the die to the heat sink.

The high temperature used to attach a die to a flange can damage ordeform plastic, however ceramic materials are able to withstand thishigh temperature. A circuit package that employs a ceramic frame can,therefore, be assembled prior to the attachment of the die. A lid isthen adhered to the frame, such as by epoxy.

To match the coefficients of thermal expansion of ceramic frames,flanges for these frames are typically made of a copper-tungsten alloyby a powder metallurgy infiltration process. This process is veryexpensive, and the thermal conductivity of the resulting alloy islimited. Improved thermal conductivity can be achieved through the useof copper-molybdenum-copper laminated flanges fabricated by aninfiltration process followed by a lamination process, however theseprocesses are very expensive.

Alternatively, the die can be attached to the flange prior to attachingthe frame and lid. This approach enables use of low-temperature plasticfor the frame, however adhesives used to attach the frame to the flangeand to the lid perform less than satisfactorily. These adhesives oftencreate imperfect seals or permit gaps to open during use of the circuitpackage. Furthermore, users of circuit packages prefer not to inventoryflanges, frames and lids and assemble these pieces after attaching dieto flanges.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a low piece-count, low cost circuitpackage that can withstand high die-attach temperatures and can providea hermetically sealed air cavity for a die, without the use ofadhesives. The circuit package design employs a number of mechanicalfeatures and compositions to achieve this hermeticity and temperaturetolerance. This combination also provides a circuit package thatexhibits better electrical and thermal conductivity and mechanicalintegrity than conventional circuit packages.

The circuit package includes two parts: a flange/frame/lead(s)combination and a lid. The leads extend through sidewalls of theopen-top frame. The flange includes a die-attach area surrounded by theframe. A seal is applied inside the frame along boundaries between theflange and the frame and between the leads and the frame. Materials forthe frame (thermoplastic, preferably liquid crystal polymer) and seal(preferably epoxy) are formulated to withstand die-attach temperatures.Once a die is attached to the flange and the die is electrically bondedto the leads, the lid is welded to the frame to seal the air cavityaround the die.

The flange, frame and leads of the circuit package include one or morestructural features to maintain mechanical integrity of the circuitpackage without use of adhesives. These features mechanically lock theflange and leads to the frame at their respective junctions.

In one embodiment, the flange defines a frame retention featuresurrounding the die-attach area. The retention feature can be, forexample, a groove or ridge that includes a dovetail or other undercutcross-sectional shape. The thermoplastic frame is molded to the flange,such as by injection molding. During molding, a portion of the frameforms a key that freezes in or around the retention feature, thusmechanically securing the frame to the flange.

In another embodiment, each lead includes one or more lead retentionfeatures to secure the lead to the frame. During the molding operation,the frame is also molded around the lead, which extends from outside theframe, through a sidewall of the frame, into the air cavity area.

One lead retention feature defines at least one hole through the lead.During molding, some frame thermoplastic flows through, and then freezesin, the hole, thus locking the lead within the frame.

Another lead retention feature provides a hooked edge, ridge or otherstructure on or near the end of the lead that resides within the aircavity area. This structure is not co-planar with the lead. Duringmolding, some frame thermoplastic freezes against an outward-facingportion of this structure, thereby creating a mechanical barrier thatprevents the lead from being pulled out of the frame.

Compositions of the flange, frame and leads provide matchingcoefficients of thermal expansion (CTE), thus reducing stress on therespective junctions between these parts. These compositions alsoprovide good thermal conductivity by the flange and good electricalconductivity by the leads and the flange. In one embodiment, the flangeis made with a high copper content, augmented by a small amount ofzirconium, silver or other material. In another embodiment, the leadsare made with a high copper content, augmented by a small amount ofiron, phosphorus, zinc and/or other material. In yet another embodiment,graphite flakes in the frame form a moisture barrier. These graphiteflakes and other additives match the CTE of the frame to the CTE of theflange. An optional film can be applied to the exterior or interior ofthe frame and/or lid to further reduce moisture infiltration into theair cavity.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other features, advantages, aspects and embodiments of thepresent invention will become more apparent to those skilled in the artfrom the following detailed description of an embodiment of the presentinvention when taken with reference to the accompanying drawings, inwhich:

FIG. 1 is a perspective view of a circuit package, without a lid,according to one embodiment of the present invention;

FIG. 2 is a perspective view of the circuit package of FIG. 1 with a lidattached thereto;

FIG. 3 is a top view of a strip of lead frames, such as those used tomanufacture the circuit package of FIG. 1;

FIG. 4A is a top view of the strip of lead frames of FIG. 3 after framesand flanges have been molded thereto;

FIG. 4B is a perspective view of one lead frame of the strip shown inFIG. 4A;

FIG. 5A is a cut-away view of a portion of the circuit package of FIG.1;

FIG. 5B is a cut-away view of a portion of an alternative embodiment ofthe circuit package of FIG. 1;

FIGS. 6A-C are cross-sectional views of the flange of the circuitpackage of FIG. 1 showing three stages of manufacture thereof;

FIGS. 7A-D are schematic drawings of a die being attached to the flangeof the circuit package of FIG. 1;

FIG. 8A is a detailed perspective view of the leads of the circuitpackage of FIG. 1;

FIG. 8B is a cross-sectional diagram of several alternative embodimentsof the leads of the circuit package of FIG. 1;

FIG. 9 is a schematic cross-sectional diagram of a portion of the frameof the circuit package of FIG. 1;

FIG. 10 is an enlarged view of a portion of the circuit package of FIG.1 showing seals;

FIGS. 11A and 11B are cross-sectional views of the circuit package ofFIG. 1 showing two embodiments of the seals of FIG. 10;

FIG. 12 is a graph showing a relationship between viscosity and shearrate of one embodiment of a material suitable for use as the seal ofFIGS. 10, 11A and 11B;

FIG. 13A is a perspective view of a lid for the circuit package of FIG.1;

FIG. 13B is a cross-sectional view of a portion of the lid of FIG. 13A,according to one embodiment; and

FIG. 14 is a flowchart of a process of making the circuit package ofFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

This application is a divisional of U.S. application Ser. No. 10/920,857filed Aug. 18, 2004 and entitled THERMOPLASTIC MATERIAL, which is adivisional of U.S. application Ser. No. 10/767,309 filed Jan. 29, 2004and now U.S. Pat. No. 6,867,367 and entitled PACKAGE FOR INTEGRATEDCIRCUIT DIE, which claims the benefit of provisional Application No.60/443,470 filed Jan. 29, 2003, all of which are hereby incorporated byreference herein.

The present invention provides a low piece-count circuit package thatcan withstand high die-attach temperatures and can provide ahermetically sealed air cavity for a die, without the use of adhesives.FIG. 1 shows an exemplary circuit package 100, according to oneembodiment of the present invention. For clarity, the circuit package100 is shown without a lid. The circuit package 100 includes a flange102, a frame 104 and two leads 106 and 108. The frame 104 electricallyinsulates the leads 106 and 108 from the flange 102 and each other. Adie 110 is attached to a die-attach area 112, such as by eutectic solder114. For clarity, FIG. 1 shows only one die, although typically two ormore die can be attached to the die-attach area 112.

The eutectic solder 114 electrically bonds the die 110 to the flange102. The eutectic solder 114 also conducts heat away from the die 110 tothe flange 102. In use, the flange 102 is typically mounted to a heatsink (not shown) by bolts (not shown) extending through slots 116 and118. The die 110 is electrically bonded to the leads 106 and 108, suchas by wires 120 and 122. These wires 120 and 122 are preferablyultrasonically bonded to the leads 106 and 108. Although one die 110 andtwo leads 106 and 108 are shown, more die and/or leads can be used. FIG.2 shows the circuit package 100 after a lid 200 has been attachedthereto, as described in more detail below.

The circuit package 100 employs a number of mechanical features andcompositions to hermetically seal the die within the air cavity and totolerate high temperatures. As previously noted, this combination alsoenables the circuit package 100 to exhibit enhanced electrical andthermal conductivity and mechanical integrity. The following descriptionbegins with an overview of a process for manufacturing the circuitpackage 100. There then follows a detailed description of the flange 102and its manufacture. This is followed by detailed descriptions of theleads 106 and 108, composition of the liquid crystal polymer used forthe frame 104, a seal applied inside the frame 104, the lid 200 and aprocess for manufacturing the circuit package 100.

Manufacturing Overview

Circuit packages 100, according to the present invention, are preferablyfabricated in strips or on reels, similar to conventional circuitpackages. FIG. 3 shows a strip 300 of lead frames, such as lead frames302 and 304. Each lead frame includes two leads, such as those shown at306 and 308. In one embodiment, when the lead frame strip 300 is stampedor etched, holes are created through the leads 306 and 308. Examples ofthese holes are shown at 310. These holes 310 are used to lock a frameto the leads 306 and 308, as described in detail below.

After the lead frame strip 300 is made, a frame is molded, preferably byinjection molding, to each lead frame of the lead frame strip. FIG. 4Ashows the lead frame strip 300A after frames, such as frame 400, havebeen molded to the lead frames. FIG. 4B shows one complete lead frame404. Lead frames can be supplied individually or in strips or reels tosubsequent manufacturers, who attach die to them.

Flange

The flange 102 forms a base, to which other parts of the present circuitpackage are attached. In addition, the flange 102 typically conductsheat from a die to a heat sink and electrically bonds one terminal ofthe die to a circuit board. The flange 102 is preferably made of ahigh-copper alloy (at least about 50% copper) to provide high electricaland thermal conductivity and to resist annealing at die-attachtemperatures. The alloy preferably includes at least one trace metal.The flange 102 preferably comprises at least about 98% copper andbetween about 0.05% and about 1.5% zirconium, although other high-copperratios are acceptable. The flange 102 more preferably comprises about99.9% copper and about 0.1% zirconium. The flange 102 is preferablyelectroplated with about 100 micro-inches of nickel to form a diffusionbarrier layer and about 65 micro-inches of gold to facilitate solderingthe die 110 to the flange.

Alternatively, the flange 102 comprises at least about 99.5% copper andabout 0.085% silver, although other high-copper ratios are acceptable.Zirconium is preferred to silver, because an alloy made with zirconiumcan contain a higher copper content and thus provide better thermal andelectrical conductivity than if it is made with silver. Thecopper-zirconium alloy provides a flange with a thermal conductivitysuperior to prior art copper-tungsten and copper-molybdenum-copperflanges, which enables a circuit package that employs such a flange or adie attached to such a flange to dissipate more power than prior artpackages. In addition, the copper-zirconium alloy has a higher annealingtemperature than most high-copper alloys and is subject to less warpageas a result of being heated to die-attach temperatures.

As previously noted, the frame 104 is molded, preferably by injectionmolding, to the flange 102. As a result of this molding, the frameadheres to the flange 102, although this adhesion is typically imperfectand subject to breakdown due to the heat of soldering and operation ofthe die. To overcome this problem, the flange 102 preferably includes amechanical feature to mechanically interlock the frame 104 and theflange.

This feature is shown in FIG. 5A, which is a cut-away view of a portionthe circuit package 100 discussed above with reference to FIG. 1. Theflange 102 defines a frame retention feature 500, which is used tomechanically interlock the frame 104 and the flange. When the frame 104is molded to the flange 102, some of the frame material flows into, andthen freezes in, the frame retention feature 500, forming a key 502. Theframe retention feature 500 has a cross-sectional profile, and the key502 takes on a complementary profile. Thus, the frozen key 502mechanically interlocks with the frame retention feature 500, therebypreventing the frame 104 from being pulled away from the flange 102without requiring an adhesive to be added between the frame and theflange.

The frame retention feature 500 includes at least one undercut portion.In cross-section, the retention feature 500 is preferably a dovetailshape, which defines two undercut portions 504 and 506. Othercross-section shapes, such as a T, L or lollipop, are acceptable.

Although the frame retention feature 500 shown in FIG. 5A is depressedbelow the adjacent surface of the flange 102, the frame retentionfeature can alternatively stand proud of the adjacent surface, as shownin FIG. 5B. Alternative flange 102A defines a frame retention feature500A that stand proud of the adjacent surface. When a frame 104A ismolded to the flange 102A, some of the frame material flows around, andfreezes below, undercut portions 504A and 506A of the frame retentionfeature 500A. In this case, the frame 104A defines a key 502A that iswithin the frame 104A.

Returning to FIG. 5A, the frame retention feature 500 is formed in theflange 102 by a series of progressive stampings. FIGS. 6A-C showcross-sections of the frame retention feature 500 at various stages ofmanufacture. FIG. 6A shows a flange blank 102B before the frameretention feature 500 has been made.

FIG. 6B shows a flange blank 102C after a first rectangularcross-section groove 600 has been coined in the flange blank. Thiscoining operation creates walls 602 and 604 in the groove 600. Thegroove 600 is preferably about 0.02 inches wide (dimension A) andpreferably about 0.02 inches deep (dimension B).

FIG. 6C shows the flange 102 after a second rectangular cross-sectiongroove 606 has been coined over the first groove 600. The second coiningoperation deforms the walls 602 and 604 (FIG. 6B), causing them tocollapse slightly near the top of the groove. Deformed walls 602A and604A form the undercuts 504 and 506 discussed above, with reference toFIG. 5A. The second groove 606 is preferably about 0.05 inches wide(dimension D) and preferably about 0.01 inches deep (dimension C). Theresulting dovetail shape has a smaller dimension of about 0.007 inches(dimension E), leaving an overhang of about 0.0065 inches (dimension F).The overhang (F) is preferably at least about 0.005 inches for theliquid crystal polymer (described below) used for the frame 104.

All these dimensions can vary depending on the size, material andtemperature of the flange 102, the size, material and temperature of theframe 104, the desired strength of the junction between the flange andthe frame, cost or other factors that are now well within the skill ofan ordinary practitioner.

The flange 102 also includes a mechanical feature to ensure a good heattransfer connection between the flange and a heat sink. Heat sinks aretypically machined flat on one surface. To provide a goodheat-conducting junction between a circuit package and a heat sink, thecircuit package should lie flat against this flat surface, without gapstherebetween.

The stamping operations (described above) performed to create the frameretention feature 500 can deform the bottom of the flange 102, therebypreventing the circuit package from lying flat against the heat sink. Toameliorate this deformation, the bottom of the flange 102 is preferablylapped after the stamping operations. In addition or alternatively,increasing the thickness (dimension G in FIG. 5A), preferably to about0.125 inches, can reduce the amount of deformation caused by thestamping operations and can eliminate the need to lap the bottom of theflange 102.

Differences in coefficients of thermal expansion (CTE) between the die110 and the flange 102 can deform the flange when the die is soldered tothe flange. FIGS. 7A-D schematically illustrate this circumstance. FIG.7A shows a flange 102D with a flat bottom 700 and a die 110 that has notyet been soldered to the flange. Solder material 114A has not yet beenmelted. The CTE of a copper/zirconium flange is approximately 17 ppm/°C., whereas the CTE of a silicon die is approximately 2.8 ppm/° C. Asthe die 110 and flange 102D are heated to solder the die to the flange,the die and flange expand.

Later, and shown in FIG. 7B, as the die 110 and flange 102D cool,eutectic solder 114B suddenly hardens, but the flange and die continuedto cool and contract. The eutectic solder 114B is very hard and not veryductile. Therefore, the contraction of the top surface 702 of the flange102D is constrained by the die 110, which has a much smaller CTE thanthe flange. As a result, the top surface 702 of the flange 102Dcontracts less than the bottom surface 700, causing the bottom surfaceto take on a concave shape, which can leave a gap when the flange ismounted to a heat sink.

To counteract the tendency of the flange 102 to take on a concave shapeafter soldering, the flange is preferably given a slightly convex shapeprior to the soldering. FIG. 7C shows the flange 102 before the die 110is soldered thereto. The bottom surface 704 of the flange 102 is given ashape, whose convexity (dimension H) is greater than the amount ofconcavity that would be introduced by soldering. In one embodiment, thebottom surface 704 is convex at least by about 0.0001 inches over thelength of the flange. In another embodiment, the bottom surface 704 isconvex by between about 0.0005 inches and about 0.0010 inches. Thisamount can be varied depending on various factors, such as the solderingtechnique used, the number, size and placement of die soldered to theflange, the length, width and thickness of the flange and thecomposition of the flange. Conventional flanges are typically about0.040 or 0.062 inches thick. A flange thickness (dimension G in FIG. 5A)of preferably about 0.125 inches can reduce the amount of deformationcaused by the soldering. The bottom surface convexity is preferablyimparted by a coining process, although other processes, such assanding, bending, casting or forging are acceptable.

FIG. 7D shows the flange 102 after the die 110 has been soldered theretoand both have cooled. The bottom surface 704 preferably has a slightlyconvex shape. When the flange 102 is mounted to a heat sink, forcesapplied by mounting screws to the flange (as indicated by arrows 706 and708) flatten the flange against heat sink and creates a goodheat-transfer junction between the flange and heat sink.

As previously noted, the flange 102 includes a generally planardie-attach area 112, to which the die 110 is soldered, epoxied orotherwise attached. The die-attach area 112 is preferably flat to withinabout 0.001 inches per inch, and more preferably to within about 0.0005inches per inch, to facilitate a good eutectic solder connection betweenthe die 110 and the die-attach area. In addition, the die-attach areasurface roughness is preferably less than about 30 micro-inches foreutectic soldering. The surface roughness of the bottom of the flange102 is preferably less than about 64 micro-inches to facilitate makinggood head-conducting contact with a heat sink. If an adhesive, such asepoxy, is used to attach the die 110 to the die-attach area 112, thedie-attach area is preferably flat to within about 0.005 inches per inchand smooth to within about 64 micro-inches.

Also as previously noted, the flange 102 includes mounting slots 116 and118. Alternatively, the flange 102 can include threaded or unthreadedmounting holes. In these cases, the flange 102 can be mounted to a heatsink or other substrate by bolts or other fasteners extending throughthese openings. Alternatively, the flange 102 can be soldered to a heatsink or other substrate, obviating the need for mounting slots.

Leads

As previously noted with reference to FIG. 1, the frame 104 is molded,preferably by injection molding, to the flange 102 and to the leads 106and 108. During the molding operation, the frame 104 is moldedpreferably around the leads 106 and 108, which extend from outside theframe, through sidewalls of the frame, into the air cavity area. As aresult of this molding, the frame adheres to the leads 106 and 108,although this adhesion is typically imperfect and subject to breakdowndue to the heat of soldering and operation of the die. To overcome thisproblem, each lead 106 and 108 preferably includes one or more leadretention features to secure the lead to the frame 104.

One lead retention feature defines at least one hole 310 through eachlead, as shown in FIG. 8. As previously noted, the holes 310 are formedin the leads 106 and 108 when the lead frame 300 (FIG. 3) is stamped oretched. Preferably, each lead 106 and 108 includes a plurality ofpreferably rectangular holes 310 arranged in a line where the frame 104will contact the lead. During molding, some frame thermoplastic flowsinto, and then freezes in, the holes 310, thus mechanically locking thelead 106 or 108 within the frame 104 and preventing the lead from beingpulled out of the frame without requiring an adhesive to be addedbetween the lead and the frame. As shown in FIG. 5A, the holes 310 arepreferably completely covered by the frame 104.

Electrical conductivity of the leads 106 and 108 contributes to theoverall performance of the circuit package 100. The conductivity of alead 106 or 108 is proportional to a lateral, i.e. approximatelyperpendicular to the direction of current flow through the lead,cross-sectional area of the lead. Because the holes 310 reduce thiscross-sectional area (see section line B-B in FIG. 8), the number,placement, size and shape of the holes can be selected to minimize theloss in effective conductivity of the leads 106 and 108. Preferably, theholes 310 reduce the cross-sectional area of the lead at most by about25%, although this reduction can be greater if the conductivity of theresulting lead meets design criteria.

Rectangular holes 310 maximize the amount of frame thermoplastic thatcan freeze and lock the leads 106 and 108, while minimizing thereduction in conductivity of the leads. The longer dimensions of therectangular holes 310 are preferably aligned parallel to the directionof current flow through the leads 106 and 108. Depending on thethickness of the sidewalls of the frame 104, the holes 310 can besquare.

Another lead retention feature, shown in FIG. 8, provides a hooked orbent (hereinafter collectively “hooked”) edge 800, ridge, depression orother structure on or near the end of the lead 106 or 108 that resideswithin the air cavity area. This structure is not co-planar with thelead. As can be seen in FIG. 5A, during molding, some framethermoplastic freezes in or against an outward-facing portion of thisstructure, thereby creating a mechanical barrier that prevents the lead106 from being pulled out of the frame 104. The hooked edge 800, ridgeor other structure is formed in the leads 106 and 108 when the leadframe 300 (FIG. 3) is stamped. Although a hooked edge 800 is thepreferred embodiment for this lead retention feature, other shapes canbe used. Examples of some acceptable shapes are shown in cross-sectionin FIG. 8B at 800A-F.

As previously discussed with reference to FIG. 1, the leads 106 and 108are used to electrically connect the die 110 to a circuit board or thelike. The leads 106 and 108 are made of a high-copper alloy (at least50% copper) to provide good electrical conductivity and to match the CTEof the frame 104. High-copper leads provide electrical conductivity thatis superior to prior art leads, which typically comprise 42% nickel and55% iron (commonly known as Alloy 42). In addition, the leads 106 and108 are preferably electroplated with about 100 micro-inches of nickelto form a diffusion barrier layer and about 65 micro-inches of gold tofacilitate wirebonding or lead soldering the leads.

The leads 106 and 108 are preferably made of an alloy of between about2.1% and about 2.6% iron, between about 0.015% and about 0.15%phosphorus, between about 0.05% and about 0.2% zinc, with the balancecopper. Other ratios of these materials are, however, acceptable. Theleads 106 and 108 are more preferably made of about 97.5% copper, about2.35% iron, about 0.3% phosphorus and about 0.12% zinc. Such an alloy isavailable from Olin Corporation under the UNS designation C19400.

Many alternative compositions for the leads 106 and 108 are acceptable.One such alternative includes about 99.9% copper and about 0.1%zirconium. Such an alloy is available from Olin Corporation under UNSdesignation C15100. Other ratios of these materials are, however,acceptable. For example, an alloy made of between about 0.05% and about0.15% zirconium, with the balance copper, is also acceptable.

Another alternative composition for the leads 106 and 108 includesbetween about 1% and about 2% iron, between about 0.01% and about 0.035%phosphorus, between about 0.3% and about 1.3% cobalt, between about 0.1%and about 1% tin and the balance copper. The preferred amount of copperin this composition is 97%. Such an alloy is available from OlinCorporation under UNS designation C19500.

Another alternative composition for the leads 106 and 108 includesbetween about 0.3% and about 1.2% iron, between about 0.1% and about0.4% phosphorus, between about 0.01% and about 0.2% magnesium, and thebalance copper. The preferred formulation in this alternativecomposition is about 0.6% iron, about 0.2% phosphorus, about 0.05%magnesium and about 99% copper. Such an alloy is available from OlinCorporation under UNS designation C19700.

Another alternative composition for the leads 106 and 108 includesbetween about 1.7% and about 2.3% tin, between about 0.1% and about 0.4%nickel, up to about 0.15% phosphorus and the balance copper. Such analloy is available from Mitsubishi Electric Corporation under UNSdesignation C50710.

Yet another alternative composition for the leads 106 and 108 includesbetween about 0.05% and about 1.5% iron, between about 0.025% and about0.04% phosphorus and the balance copper. Such an alloy is available fromKobe Steel, Ltd. under UNS designation C19210.

Yet another alternative composition for the leads 106 and 108 includesbetween about 0.5% and about 0.15% iron, between about 0.5% and about1.5% tin, between about 0.01% and about 0.035% phosphorus and thebalance copper. Such an alloy is available from Mitsubishi ShintoCompany, Ltd. under UNS designation C19520.

Another alternative composition for the leads 106 and 108 includesbetween about 0.15% and about 0.4% chromium, between about 0.01% andabout 0.4% titanium, between about 0.02% and about 0.07% silicon and thebalance copper. Such an alloy is available from Wieland Werke under UNSdesignation C18070.

Yet another alternative composition for the leads 106 and 108 includesbetween about 0.8% and about 1.8% nickel, between about 0.15% and about0.35% silicon, between about 0.01% and about 0.05% phosphorus and thebalance copper. Such an alloy is available from Poong San MetalCorporation under UNS designation C19010.

Another alternative composition for the leads 106 and 108 includesbetween about 2.0% and about 4.8% nickel, between about 0.2% and about1.4% silicon, between about 0.05% and about 0.45% magnesium and thebalance copper. The preferred formulation in this alternativecomposition is about 3.0% nickel, about 0.65% silicon, about 0.15%magnesium and about 96.2% copper. Such an alloy is available from OlinCorporation under UNS designation C70250.

Yet another alternative composition for the leads 106 and 108 includesbetween about 0.3% and about 0.4% chromium, between about 0.2% and about0.3% tin, between about 0.15% and about 0.25% zinc and the balancecopper. Such an alloy is available from Furukawa Electric under UNSdesignation EFTEC-64T.

Another alternative composition for the leads 106 and 108 includesbetween about 2.7% and about 3.7% nickel, between about 0.2% and about1.2% silicon, between about 0.1% and about 0.5% zinc and the balancecopper. Such an alloy is available from Kobe Steel, Ltd. under UNSdesignation KLF-25.

Yet another alternative composition for the leads 106 and 108 includesbetween about 1.9% and about 2.9% nickel, between about 0.2% and about0.6% silicon, between about 0.1% and about 0.2% phosphorus and thebalance copper. Such an alloy is available from Mitsubishi ElectricCorporation under UNS designation MF224.

Frame

As noted above, with respect to FIG. 5A, the frame 104 is made ofinjection molded thermoplastic and is molded to the flange 102 and tothe leads 106 and 108. The material of the flange 102 preferablyincludes a liquid crystal polymer (LCP) that can withstand die-attachtemperatures (280-330° C. for AuSn soldering or 390-420° C. for AuSisoldering). Conventional LCPs melt at temperatures between about 300° C.and about 330° C. The frame 104 preferably includes base resins andcompounds to raise its melting temperature, adjust its coefficient ofthermal expansion (CTE) and reduce its permeability to moisture. Forconvenience, the material of the frame 104, including the resins andcompounds, is referred to here herein as a “thermoplastic compound” or“frame material.”

An example of an acceptable resin is one that includespara-hydroxybenzoic acid, bisphenol and phthalic acid. Anotheracceptable formulation includes a copolymer of p-hydroxybenzoic acid(HBA) and 6-hydroxy-2-naphthoic acid (HNA). Other acceptableformulations include terpolymers of HBA, 4-4-bisphenol (BP) andterephthalic acid (TA).

FIG. 9 is a schematic cross-section diagram of the frame 104 showingsome of the compounds in the thermoplastic compound. Filler particlesare preferably added to the LCP to modify its CTE to more closely matchthe CTE of the leads 106 and 108 (approximately 17 ppm/° C.) and todisrupt the anisotropy of the thermoplastic compound in the frame 102.The CTE of the frame material is preferably adjusted to be within about60% of the CTE of the leads 106 and 108. Spherical balls of minerals900, such as talc, preferably about 2 to 3 microns in diameter, can beadded to the LCP at concentrations of about 30% to about 40%. Such acomposite has a CTE of about 7 ppm/° C. to 22 ppm/° C.

Graphite is preferably added to the LCP to reduce moisture infiltration.This graphite is preferably in the form of generally planar graphiteflakes 904 (shown edge-on in FIG. 9), however other forms of graphite,such as balls or chunks, are also acceptable. In addition, the graphiteflakes 904 can warp somewhat during injection, etc., withoutsignificantly altering their effectiveness. The term “generally planargraphite flakes” includes such flakes that have been deformed.

The graphite flakes 904 preferably form layers, preferably roughlyparallel to exterior surfaces of the frame 104, thus creating tortuouspaths 906 for moisture infiltration. Even if the layers are not parallelto the exterior surfaces, the presence of the graphite inhibits moistureinfiltration. The graphite flakes 904 also adjust the CTE of the LCP tomore closely match the copper alloy of the leads 106 and 108. The framematerial contains between about 10% and about 70% graphite flakes,preferably between about 40% and about 50%.

As an alternative to graphite flakes, glass fiber 1202 can be added tothe LCP to increase rigidity and adjust CPE of the resultingthermoplastic compound. In this embodiment, the frame materialpreferably contains between about 30% and about 50% glass fiber.

As another alternative, or in addition, to the graphite flakes, othercompounds can be added to the LCP, such as iron powder based absorbers,molecular sieve filters (zeolites) and calcium oxide (CaO). Suitablezeolites are available from Sud-Chemie, Inc.

The frame material is preferably pre-dried, preferably to less thanabout 0.008% moisture content, before injection molding. In addition,injection times should be kept short, preferably less than about 0.2seconds. Injection shot sizes should be kept small, preferably less thanabout 2 grams, to minimize residence time of the thermoplastic compoundin the injection molder barrel. A gate at the injection site preferablyrestricts flow of the thermoplastic compound, thereby increasing shearon the thermoplastic compound, to orient the polymer chains and thegraphite flakes 904. The thermoplastic compound is preferably injectedat a corner of the frame 104 or between the leads 106 and 108. Tominimize the amount of stress in the resulting frame, a minimum moldtemperature of about 250° F. is preferably maintained during the moldingoperation.

A moisture barrier film is preferably applied to the exterior surfacesof the frame 104 to further reduce moisture infiltration. Alternatively,the film can be applied to the interior of the frame 104. Acceptablematerials include amine-based epoxies available from PPG Industriesunder the trade name Bairocade, polymer-Al films and polymer-ceramicfilms.

Seal

The frame retention feature 500 (FIG. 5A) and the lead retention feature800 (FIG. 8A) provide good mechanical connections that inhibitinfiltration of moisture and atmospheric gases. In addition, the frame104 preferably includes constituents and an exterior or interior film toreduce this infiltration. To further reduce infiltration, as illustratedin FIG. 10, seals 1000 and 1002 are preferably applied inside the frame104 along edges of the frame 104, where the frame meets the leads 106and 108 and where the frame meets the flange 102. As shown incross-section in FIG. 11A, seal 1002 is effective to preventinfiltration between the flange 102 and the frame 104 and to preventinfiltration between the frame 104 and the lead 108. Alternatively, asshown in FIG. 11B, two seals, 1002A and 1002B can be used instead of theone seal 1002.

To promote good adhesion of the sealant to the frame material, the framematerial is preferably cleaned prior to application of the sealant.Plasma cleaning, as is well-known in the art, where oxygen is thepredominant medium produces acceptable results. Alternatively, the framematerial can be cleaned with solvents or by etching. A 0.008 inchinside-diameter (ID) needle is preferably used to dispense material forseal 1000, and a 0.010 inch ID needle is preferably used to dispensematerial for seal 1002. Other needle sizes can, of course, be used,depending on the size of the desired bead. To minimize air bubbles inthe sealant, a positive displacement, auger pump is used to pump thesealant to the needles.

The sealant material preferably has a viscosity between about 58 Pa·sand about 128 Pa·s at a shear rate of about 0.95 per second, and aviscosity between about 12 Pa·s and about 30 Pa·s at a shear rate ofabout 9.5 per second. FIG. 12 contains a graph 1200 showing a preferredrelationship between viscosity and shear rate. A low viscosity ispreferred at high shear rates, so the material can be dispensed quickly.However, a high viscosity is preferred at low shear rates, so, once thematerial has been dispensed, it does not run.

The material preferably has a cason viscosity between about 3 Pa·s andabout 7.4 Pa·s. The material's thixotropic index is preferably betweenabout 3.5 and about 4.6.

Suitable materials for the seals 1000 and 1002 include epoxies,silicones and conformal coatings. A suitable sealant includes betweenabout 40% and about 60% solvent (such as 2-butyl acetate) and betweenabout 40% and about 60% epoxy or silicone resin. The epoxy can be, forexample, bis-phenol A or a cyclic aliphatic epoxide resin. Suitablehardeners include amine hardeners. Alternatively, the seal can be madeof Paralyne D or Parylene HT, available from Cookson Electronics.

Lid

The lid 200 is attached to the frame 104 after the die 110 has beenattached to the flange 102 and electrically bonded to the leads 106 and108. A suitable lid 200 is shown in FIGS. 13A and 13B. The lid 200 ispreferably ultrasonically welded to the frame 104, preferably using awelding signal having a frequency between about 50 kHz and about 60 kHzand an amplitude less than about 100 microns (more preferably less than60 microns). Alternatively, the lid 200 can be laser welded or heatwelded to the frame 104.

Conventional ultrasonic plastic welding techniques have not been used toseal lids to circuit packages, because these conventional techniques uselower welding frequencies, which result in higher amplitudes that candamage wirebond assemblies mounted in circuit packages. The higherwelding frequencies of the present invention result in lower amplitudesand, consequently, do not damage wirebond assemblies. Conventionally,lids are attached to circuit packages with epoxy adhesives.Advantageously, ultrasonic welding can be accomplished in much less time(approximately 250 mSec.) than epoxy cure times (apx. 2 hours).

There is preferably an interference fit between the lid 200 and theframe 104, so portions of both the lid and frame melt and fuse togetherduring the ultrasonic welding. As shown in the cross-section in FIG.13B, the lid 200 preferably includes a lip 1300 that melts and fuseswith the frame 104. The lid 200 is preferably made of the same materialas the frame 104, as described above. In addition, a moisture barrierfilm is preferably applied to the lid 200, as described above withrespect to the frame 104.

Manufacturing Details

Details of the flange 102, frame 104, leads 106 and 108 and lid 200 ofthe circuit package 100, including materials and processes used tofabricate these parts, have been described in detail above. FIG. 14contains a simplified flowchart illustrating a process, by which thecircuit package 100 can be made and used.

At 1400, a first high-copper alloy is made for the leads 106 and 108. At1402, lead frames are fabricated from the first high-copper alloy madeat 1400. At 1404, holes 310 are punched, etched or otherwise made inlead frame 300 to create one of the lead retention features. At 1406,the ends of the leads 106 and 108 are curled, bent, stamped or etched onthe lead frame 300 to create the other lead retention feature 800.

At 1408, a second high-copper alloy is made for the flange 102. At 1410,the flange 102 is fabricated from the second high-copper alloy made at1408. At 1412, the frame retention feature 500 is coined in the flange102 by a progressive stamping process. Optionally, at 1414, the bottomof the flange 102 is lapped. At 1416, the bottom of the flange 102 ismade convex.

At 1418, graphite flakes, talc and/or glass fiber is added to liquidcrystal polymer to make frame material. At 1420, the frame material isdried. At 1422, the frame 104 is molded to the flange 102 and the leads106 and 108.

At 1424, the interior of the frame 104 and flange 102, i.e. the aircavity area, is cleaned. At 1426, the sealant is applied to seal theboundaries between the frame 104 and the flange 102 and between theframe and the leads 106 and 108.

At 1428, the die 110 is attached to the flange 102. At 1430, the die 110is ultrasonically wirebonded to the leads 106 and 108. At 1432, the lid200 is ultrasonically welded to the frame 104.

While the invention has been described with reference to a preferredembodiment, those skilled in the art will understand and appreciate thatvariations can be made while still remaining within the spirit and scopeof the present invention, as described in the appended claims. Forexample, lower power die can be attached to the flange with epoxy orother adhesives, rather than being soldered thereto. In addition, as iswell known in the art, alloys typically contain small amounts ofimpurities, so constituents described herein do not necessarily total100%.

Although the frame retention feature 500 and the convexity H of theflange bottom have been described in the context of a high-copper flange102, these innovations are also applicable to conventional flanges andflanges made of other materials. Although the lead retention features310, 800 and their respective alternatives have been described in thecontext of high-copper leads, these innovations are also applicable toconventional leads and leads made of other materials. Although the framematerial has been described in the context of a circuit package 100,this material can be advantageously used in other contexts, such asthose that require a material that can withstand high temperatures.Examples of other applications of the frame material includehigh-temperature laminates in printed circuit boards (PCBs) and socketsfor electronic components, cables, PCBs and the like.

1. A structure comprising a liquid crystal polymer (LCP) material, thematerial having a plurality of generally planar graphite flakeinclusions; wherein the structure has a surface, and the graphite flakesare essentially organized into layers essentially parallel to saidsurface.
 2. The structure of claim 1, wherein the layers of graphiteflakes form tortuous paths therebetween that inhibit the infiltration ofmoisture into the material.
 3. The structure of claim 1, wherein thestructure includes a plurality of surfaces, and the layers of graphiteflakes are essentially parallel to at least one of the surfaces.
 4. Thestructure of claim 1, wherein the material comprises between about 10%and about 70% graphite flakes.
 5. The structure of claim 4, wherein thematerial comprises between about 40% and about 50% graphite flakes. 6.The structure of claim 1, wherein the concentration of graphite flakesis selected to provide a desired coefficient of thermal expansion (CTE)of the material.
 7. The structure of claim 1, wherein the CTE of the LCPmaterial is approximately 17 ppm/° C.
 8. The structure of claim 1,wherein the melting point of the LCP is greater than 280° C.
 9. Thestructure of claim 8, wherein the melting point of the LCP is greaterthan 390° C.
 10. The structure of claim 1, wherein the LCP is formedfrom monomeric units including: p-hydroxybenzoic acid; bisphenol; andphthalic acid.
 11. The structure of claim 1, wherein the LCP comprises acopolymer of p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. 12.The structure of claim 1, wherein the LCP comprises a terpolymer ofp-hydroxybenzoic acid, 4-4-bisphenol, and terephthalic acid.
 13. Thestructure of claim 1, wherein the structure is an electronics componentpackage body.
 14. A structure comprising a liquid crystal polymer (LCP)material having a surface, and a plurality of graphite flake inclusionsconsisting essentially of generally planar graphite flakes which areessentially organized into layers essentially parallel to the surface.